Creep damage in 9-12% Cr steel notched components: Microstructural evolution and stress state dependence

Abstract

To understand the stress state dependent creep damage, following creep tests in 9–12% Cr steel notched components with different root radii, a study of associated time dependent microstructural evolution is presented in this work. Particular attention is paid to the dependence of creep damage evolution on the stress state. The average creep strain of notched component exhibits a typical three-stage characteristic (transient, steady and tertiary stages). During the transient stage, remarkable re-distribution in stress state (equivalent stress and stress triaxiality) is found. The rapid relaxation of equivalent stress near the notch root, associated with the initial coarsening of substructure would delay the nucleation or growth of cavity. By contrast, the time-dependent increase of equivalent stress combined with the relatively high stress triaxiality seems to be responsible for the higher cavity density at the center of notched component. The precipitate coarsening is heterogeneous and promoted by the high level of equivalent stress or stress triaxiality. During the steady stage, the creep damage behavior is related to the magnitude of stress state. Higher cavity density and greater reduction in hardness are mainly found in the region where both the equivalent stress and stress triaxiality are high. As a whole, both the redistribution process of stress states and their levels exhibit significant influence on microstructural evolution and creep damage behavior of notched components at elevated temperatures.